The mortar itself is a weaker kill move than utilt, and the main use for it would be the mortar slide. However, spamming the mortar slide makes you predicable, and there are ways to punish Snake for doing them so often (though I admit, as far as I know, it's very difficult in and of itself to punish).
However, since the damage and knockback are not chargeable, it's pretty easy to tell how far it'll take your opponent away. And granted, it's a smash attack, so it'll still do some respectable knockback even though its stale.
But for the record, don't spam the mortar unless you feel like a douche and start stalling using it.